I'm not the biggest fan of this not using the same pattern as we did for ALPNInformation... which would be more like this:

...
? reason: CloseReason
...

CloseReason = {
    ? byte_value: hexstring
    ? string_value: text
}

Is there a particular reason to change from that established precedent?

The main reason (pun intended) is that the QUICALPNInformation event contains 3 instances of ALPNInformation, which would make a flatter structure really hard to describe. Here, there is only one instance of the reason, so it seems of less value to create another structure type to hold it.

I won't defend that choice with much vigor if others prefer a Structured approach.

I don't have a super-strong opinion on this, as this approach also makes sense. Especially seeing another need for this in #414, we could reason this approach is the default, and ALPNInformation deviates from that for good reasons™